(1) Field of the Invention
This invention relates to digital angiographic apparatus for picking up mask images and live images, and acquiring subtraction images through a subtraction between these mask images and live images.
(2) Description of the Related Art
Conventionally, angiography with a digital angiographic apparatus of this type is performed in the following sequence. First, a mask image is picked up without injecting the patient with contrast material, and this mask image is stored in memory. Then, after injecting contrast material, a live image is picked up and stored in memory. In a subsequent process, the mask image and live image stored in memory are subjected to a subtraction to acquire a subtraction image (blood vessel image).
Mask images and live images may be picked up of a plurality of sites by shifting a positional relationship between an X-ray tube/image intensifier and the patient along or around the body axis of the patient. Then, a plurality of subtraction images of these sites are acquired by subjecting the mask images and live images to subtractions. These mask images and live images are picked up in two separate steps, and the subtraction images are acquired therefrom in a subsequent process.
Specifically, mask images are first picked up of a plurality of predetermined sites while varying a positional relationship between the patient without contrast material injected and the X-ray tube and the like. These mask images are stored in memory. Then, after injecting the patient with contrast material, live images of the same sites are picked up while shifting the positions relationship between the patient and the X-ray tube and the like along the same locus as for picking up the mask images. The live images are also stored in memory. Subsequently, the mask images and live images stored are subjected to a subtraction for each site to acquire a subtraction image of each site.
The conventional apparatus with the above construction has the following drawbacks.
According to the conventional apparatus, mask images are picked up and stored in advance and, after the injecting the patient with contrast material, live images are picked up and stored. Subtraction images are acquired in a subsequent process. Such a procedure provides a low throughput.
It is necessary to inject the patient with contrast material between mask image pickup and live image pickup. The patient could easily move during the intervening period. A movement of the patient brings about a deviation of the positional relationship between the patient and the X-ray tube and the like. This results in a slip between mask image and live image. Consequently, artifacts could appear in the subtraction images thereby produced.
In particular, where angiography is performed for a plurality of sites while varying the positional relationship between the patient and the X-ray tube and the like, mask images and live images are picked up of the respective sites in two separate steps. After picking up a mask image of one site and before picking up a live image of the same site, there occurs at least an interval of time which is a sum of time required for varying the positional relationship between the patient and the X-ray tube and the like and time required for injecting the patient with contrast material. The patient is likely to move during this extended time from mask image pickup to the live image pickup for the same site.
In addition, each live image must be picked up by shifting the positional relationship between the patient and the X-ray tube and the like along the same locus as for the mask image pickup, and must be picked up of the same site as the mask image was picked up. A disagreement tends to occur between two image pickup sites from the question of mechanical precision of a mechanism for shifting the positional relationship. The resulting subtraction image is vulnerable to artifacts.
With the conventional apparatus, therefore, a plurality of imaging sites are determined at fine intervals in performing angiography for these sites while shifting the positional relationship between the patient and the X-ray tube and the like. A plurality of mask images and live images are picked up of each of the sites determined at fine intervals. In a subsequent process, a mask image and a live image matching or close to each other are searched and retrieved. This pair of images are subjected to a subtraction to acquire a subtraction image with reduced artifacts.
However, numerous images unnecessary for medical examination are picked up when a plurality of mask images and live images are picked up of each of the sites determined at fine intervals. To pick up such unnecessary images, X rays irradiate the patient wastefully, and the patient unduly suffers increased exposure to X rays.